New Evidence That Supermassive Black Holes Eventually Suck the Life out of Big Galaxies

At the core of each large galaxy lies a supermassive black hole with the mass of 1 million suns. New research shows that these celestial hoovers do more than just devour nearby objects—they also grow to a size that eventually suppresses a galaxy’s ability to churn out new stars, effectively rendering them sterile.

Young galaxies are absolutely bursting with bright, newly formed stars. As time passes, however, star formation eventually grinds to a halt. A new study published in Nature shows that supermassive black holes play a critical role in determining when large galaxies stop producing new stars, a process known as “quenching.”

Stars form out of cold gas, so when a galaxy runs out of cold gas it’s effectively quenched. One possible way this could happen—at least for galaxies with supermassive black holes—is that the gas that pours onto a supermassive black hole triggers the production of high-energy jets. The energy released by these jets can expel cold gas out of the galaxy, causing star formation to shut down.

At least that’s the theory. This idea has been around for quite some time, but no observational evidence existed to support the alleged correlation between supermassive black holes and star formation. The new study, led by Ignacio Martín-Navarro from the University of California Santa Cruz, now fills this gap in our knowledge.

Using data collected by the Hobby-Eberly Telescope Massive Galaxy Survey, Martín-Navarro’s team analyzed the spectra of light coming from distant galaxies. This allowed them to separate and measure the varying wavelengths of light coming from these distant objects. The scientists used this data to create a historical snapshot of a galaxy’s star formation history. They then compared this history with black holes of different masses, which resulted in some striking differences—differences that correlated with black hole mass, but not the shape, size, or other properties of black holes.

“The subsequent quenching of star formation takes place earlier and more efficiently in galaxies that host higher-mass central black holes,” wrote the researchers. “The observed relation between black-hole mass and star formation efficiency applies to all generations of stars formed throughout the life of a galaxy, revealing a continuous interplay between black-hole activity and... cooling.”

As Martín-Navarro clarified in an accompanying statement, for galaxies with the same mass of stars, but with a different black hole mass in the center, “those galaxies with bigger black holes were quenched earlier and faster than those with smaller black holes.” This means that star formation will last longer in galaxies with smaller central black holes. “[...Accretion onto more massive black holes leads to more energetic feedback from active galactic nuclei, which would quench star formation faster,” he said.

It’s an exciting result, but there’s still lots of work to do. While the researchers managed to produce observational evidence showing that black hole mass can be connected to the quenching of star formation, they’re still unclear about the exact mechanical processes involved. As study co-author Aaron Romanowsky explained, “There are different ways a black hole can put energy out into the galaxy, and theorists have all kinds of ideas about how quenching happens, but there’s more work to be done to fit these new observations into the models.”

Our galaxy, the Milky Way, features its own supermassive black hole and is not immune to this process. It is currently transitioning from star-forming mode to a passive, sterile existence. Eventually, a few billion years from now, all the stars in the Milky Way will be extinguished, and the supermassive black hole at its centre will evaporate into nothing. It’s a grim prospect, but such is the way of the indifferent cosmos. [Nature]